Journal of the Department of Agriculture, Journal of the Department of Agriculture,

Western Australia, Series 4 Western Australia, Series 4

Volume 31 Number 2 1990 Article 7

1-1-1990

Causes of waterlogging Causes of waterlogging

Jim Cox

Don McFarlane

Follow this and additional works at: https://researchlibrary.agric.wa.gov.au/journal_agriculture4

Part of the Agronomy and Crop Sciences Commons, Hydrology Commons, and the Natural Resources

Management and Policy Commons

Recommended Citation Recommended Citation Cox, Jim and McFarlane, Don (1990) "Causes of waterlogging," Journal of the Department of Agriculture,

Western Australia, Series 4: Vol. 31 : No. 2 , Article 7. Available at: https://researchlibrary.agric.wa.gov.au/journal_agriculture4/vol31/iss2/7

This article is brought to you for free and open access by Research Library. It has been accepted for inclusion in Journal of the Department of Agriculture, Western Australia, Series 4 by an authorized administrator of Research Library. For more information, please contact library@dpird.wa.gov.au.

The causes of waterlogging By Jim Cox1 and Don McFarlane2

1 Research Officer, Water Authority of Western Australia, Perth

2 Research Officer, Division of Resource Management, Department of Agriculture, Albany

Waterlogging is highly variable, both between years and within paddocks. This residts in uncertainty as to whether a paddock should be cropped in a particular year, and whether different management should be used on waterlogged areas within a paddock

A study has determined what are the most important causes of waterlogged soils.

The most susceptible sites have a combination of low permeability subsoils and thin topsoils (also with a low permeability). They are on low slopes and downslope of an area that sheds water. Such sites waterlog even in years of low rainfall.

Profiles of waterlogged soils often have a mottled appearance which can be seen at any time of the year.

Waterlogging thrown areas) is affected by several factors which vary throughout the landscape.

Why waterlogging is so variable

Waterlogging is caused by a combination of excess rainfall (for the site), poor external drain­age (runoff), poor internal drainage (water movement in the soil profile) and the inability of the soil to store much water.

We studied the relative importance of these causes at Narrogin and Mt Barker to determine how to predict and manage waterlogging. The results are summarised under three sections: rainfall, soil type and landforms.

Rainfall

Narrogin

The 1985 growing season (May to October) rain­fall of 356 mm was less than the average sea­sonal rainfall of 397 mm. The average waterlog­ging intensity measured in 107 shallow wells was about 300 cm.days which is equivalent to a water level at the soil surface for 10 days. (See 'How we measured waterlogging intensity' on page 59.) About half of the wells had little or no water within 30 cm of the surface, while three had more than 1,000 cm.days of waterlogging (equivalent to 33 days with the water level at the soil surface).

Growing season rainfall in 1986 was only 267 mm (130 mm below average). Sixty per cent of the wells had little or no waterlogging. How­ever, even in this dry year four wells had more than 700 cm.days of waterlogging (equivalent to 23 days with the water level at the soil surface).

This is a common feature of waterlogging; sus­ceptible areas are waterlogged even in dry years. These areas lower the overall yield of paddocks whenever they are cropped and therefore need separate management, either drainage or they should be left uncropped.

5 8 MM. IOURNAL OF AGRICULTURE VtHSl. 199(1

How we measured waterlogging intensity

Duplex soils (sandy topsoil over a clayey sub­soil) are the most common soil type in the agri­cultural areas. Water perches on the clay sub­soil in these soils and saturates the root zone of plants from below.

When measuring waterlogging intensity we need to know how close to the soil surface these perched water levels rise, and how long the water levels stay close to the surface. We also need to know at what time of the year soils waterlog relative to the growth stage of the crop.

In this study we measured waterlogging in­tensity by summing the daily values (in cen­timetres) of groundwater levels within 30 cm of the soil surface (Figure 1). Therefore, three days with the water level 20 cm from the surface (10 cm above the 30 cm threshold) has a waterlogging intensity of 30 cm.days (3 days x 10 cm). This is equivalent to one day with the water level at the soil surface (1 day x 30 cm). This method of measuring waterlogging in­tensity is called the SEW^ index (sum of ex­cess water above 30 cm).

Perched water levels in the soil fluctuate rapidly in response to rainfall (Figure 1). Once soil profiles are wet, small amounts of rain cause the levels to rise markedly. These fluctua­tions in water level influ­ence crop growth.

Figure 1. Perched water level) in the soil fluctuate rapidly in

response to rainfall. The shaded area is the SEW index which is a measure of waterlogging intensity.

1988 Rainfall (mm)

0-20 40-60 J

w TP~

Date

Mt Barker

Average growing season (May to October) rainfall at Mt Barker is higher (470 mm) than at Narrogin (397 mm). The seasonal rainfall in 1984 was 532 mm which resulted in wide­spread waterlogging as recorded by 55 shallow wells. The average waterlogging intensity was 1074 cm.days (equivalent to 36 days with the water level at the soil surface).

Some wells recorded more than 2,500 cm.days of waterlogging (equivalent to more than 80 days with the water level at the soil surface).

In 1986, the seasonal rainfall was slightly below average (442 mm) but the average waterlog­ging intensity was still high (665 cm.days). This is equivalent to 22 days with the water level at the soil surface.

The proportion of wells with hardly any water­logging changed little between 1984 and 1986, indicating that some areas are not prone to wa­terlogging, even in wet years. Some areas appear to be so prone to waterlogging that they cannot be drained effectively, while others will

not become waterlogged even in very wet years.

The likelihood of waterlogging can be assessed from rainfall data if the previous history of wa­terlogging at a site is known. A method is out­lined in McFarlane (1985).

Soil type

The two types of soil most susceptible to wa­terlogging are duplex and heavy textured (clayey) soils, particularly when these soils occur on low slopes.

The sandy surface of duplex soils enhances water infiltration while the clay subsoil can inhibit drainage within the profile. Sandy topsoils store less water and lose less water by evaporation than do clayey soils.

Clayey soils are waterlogged when the water ponds on the surface and saturates the soil profile from the top downwards, whereas duplex soils saturate from the clay subsoil upwards.

WA. IOURNAL OF AGRICULTURE Vol. 31.1990 5 9

Duplex soils are very susceptible to waterlogging.

The soil saturates from the clay subsoil up to the surface.

Figure 2. As the thickness of the topsoil decreases, zvaterlogging intensity (as measured by the SEWM

index), increases.

Duplex soils at Narrogin and Mt Barker store hardly any water. After the break of the season, the soil profiles were saturated to within 30 cm of the soil surface once rainfall exceeded potential evaporation by only 50 mm. Water storage is highest in soils with a thick topsoil (Figure 2).

The duplex soils which were most susceptible to waterlogging had less permeable subsoils. The permeability was found to be very variable and impossible to predict. Some subsoils may be well drained where old root channels open up the clay subsoil (Figure 3).

Waterlogging was also more common in duplex soils with less permeable topsoils which result in slow downslope drainage (on top of the subsoil).

Landforms

The article The extent and cost of waterlog­ging' on page 44 of this Journal showed that floodplain areas were most susceptible to waterlogging, followed by sloping landforms with duplex soils. This section looks more closely at which landforms in individual paddocks are most susceptible to waterlogging.

Slope

At Mt Barker waterlogging was extreme in areas with little slope. Areas with a slope of 5 per cent had no waterlogging in years with between 350 and 485 mm of rainfall. In con­trast, areas with only 0.8 per cent slope had about 2,000 cm.days of waterlogging in a high rainfall year. This is equivalent to 67 days with the water level at the soil surface.

A mottled soil indicates seasonal zvaterlogging. This photo ivas taken in the Katanning area.

60 W.A JOURNAL OF AGRICULTURE VMM. 1991)

High slopes increase the amount of rain that runs off, particularly when the soil is saturated to the surface and rain cannot infiltrate. High slopes also increase the amount of lateral seepage in the soil. However, the topsoils need to be fairly permeable for this to be important.

Length and shape of the slope

Immediately after heavy rain many duplex soils will be saturated because of their low storage capacity.

As the perched water moves downslope, a drying front develops in areas where there is no inflow from above (for example, at the top of slopes and downslope of drains).

The front may take weeks to reach the bottom of slopes, in which time more rain may have resarurated the soil profile. Areas towards the bottom of long slopes are therefore highly prone to waterlogging because of the pro­longed inflow of seepage water from upslope.

The shape of the slope is important. Concave ('amphitheatre-shaped') slopes concentrate seepage waters and result in the most severe waterlogging. Waterlogging is also common where slopes decrease abruptly. Drains can prevent waterlogging in both cases.

How we can recognize sites liable to water­logging

There are a number of indicators of waterlog­ging:

• The presence of weeds such as toadrush, Phalaris species and dock that tolerate water­logging, and a predominance of grasses over broad-leaf plants. • The absence of waterlogging sensitive species such as clovers, apart from the yannini-cum subspecies (Yarloop group) and white and strawberry clovers. • The presence of red, yellow or blue-grey mottles (areas with different colours) in the soil profile. Some mottles are the result of water­logging in a past climate. • Combinations of the soil and landscape features identified above: shallow, low per­meability topsoil; low permeability subsoils; below water shedding areas or long slopes; low slopes; concave slopes.

Acknowledgements

The financial assistance of the Barley Industry Research Committee of Western Australia is gratefully acknowledged.

• -V

" • • • * & •

Clayey soils are also highly susceptible to waterlogging. The soil saturates from the top down in contrast to duplex soils which saturate from the clay subsoil upwards.

1000

0} cz

z C

— c '5 Ol _o CO re 5 — c "5 re 2 o C

I

t £ 800 >> CO

•o c 600

° 400

ui w 200

hydraulic conductivity

3.0

-1 .5

s-O.O

> co

II o o O CO si CO CD •a £ > > ^ X o

--1.5

'—-3.0 0 20 40 60 80

Distance downslope of a drain (m)

Further reading

McFarlane, D.J. (1985). Assessment of waterlogged sites. /. Agric. W. Aust. 26(4): 119-121.

Figure 3. The change in waterlogging intensity (SEWtil

index) below a drain. For the first 40 m beloiu the drain waterlogging increases. After 40 m there is no ivaterlogging because the subsoil is highly permeable (as measured by its hydraulic conductivity).

W.A. IOURNAL OF AGRICULTURE V,tl. 31.1990 6 1